This invention relates to a drive mechanism for a fuel pump of a reversible two-stroke internal combustion engine, comprising a follower roller cooperating with an actuating cam on the control shaft of the engine, a roller guide adapted for being coupled to the plunger of the pump, which guide is guided for reciprocating movement perpendicular to the control shaft and spring-biased towards the control shaft, an arm at one end of which the follower roller is rotatably supported by means of a journal pin, while the opposite end of the arm is connected to the roller guide by means of a pivot joint, the axis of which is parallel to the axis of rotation of the control shaft, and means for pivoting the arm relative to the roller guide between two end positions so as to shift the axis of the journal pin from one side of a plane through the axes of the pivot joint and the control shaft to the opposite side of that plane and vice versa.
During operation of the engine the arm supporting the follower roller is maintained in one of its end positions, in which the line of contact between the cam profile and the roller, when the associated engine piston is in its top dead centre, is laterally offset from the plane referred to. When the direction of rotation of the engine's crankshaft is to be reversed the arm is pivoted to its other end position whereby the axis of the follower roller as well as the contact point in the top dead centre position are shifted to the opposite side of said plane. It is thus possible, by means of a single, symmetric or substantially symmetric, cam and with unchanged angular position of the control shaft relative to the crankshaft, to obtain a desired fuel pump lead in both directions of rotation. A reversing mechanism for the fuel pump drive can thus be dispensed with.
From U.S. Pat. No. 2,599,479 there is known a drive mechanism of the kind referred to in which the arm supporting the follower roller is connected to an elongate reversing shaft which extends along the engine and which can be rotated about its axis between two extreme positions, each corresponding to one of the engine's directions of rotation, by means of a linkage consisting of a lever secured to the reversing shaft and a link between that lever and the roller supporting arm. Due to the geometry of the linkage the supporting arm effects, in addition to a rectilinear movement corresponding to the reciprocating movement of the roller guide, a small oscillating angular movement during each rotation of the control shaft. As a result the bearing pin between the supporting arm and the roller guide and the two bearing pins of the link effect small oscillating rotations which in practice makes it difficult to build up a sufficient lubricating film in the three bearings. A further disadvantage of the known mechanism is that the transverse component of the contact force between the roller and the cam, i.e. the force component which is perpendicular to the longitudinal axis of the supporting arm, is taken up by the link and thus exerts bending and torsional loads on the reversing shaft. The resulting deformations of the long reversing shaft lead to more or less incalculable displacements of the points in which the links are hinged to the arms and hence to undesired deviations between the fuel pump lead of individual engine cylinders. The magnitude of the transverse force components can be reduced by increasing the length of the supporting arm, but then the total height of the fuel pump above the control shaft increases and this can make it difficult to obtain the space required when an engine piston shall be pulled up from its cylinder and moved laterally above the pump in order to bring the piston clear of the engine for inspection, replacement of piston rings etc.